摘要：We estimated centroid moment tensors of earthquakes that occurred from 2003 to 2011 in and around the focal
area of the 2011 Mw 9.0 megathrust earthquake in eastern Japan. The result indicates that earthquakes occurring
before the mainshock, which included foreshocks off Miyagi, were basically interplate earthquakes with thrusttype focal mechanisms. On the other hand, the aftershocks exhibited a variety of focal mechanisms. Interplate
aftershocks with thrust focal mechanisms did not occur within the large coseismic slip area estimated from GPS
data but instead occurred in the surrounding regions. This implies that slip could no longer occur in the coseismic
slip area due to the large amount of stress release during the mainshock rupture, whereas the aftershocks in
the surrounding regions were caused by a stress concentration in these regions due to the large co-seismic slip
associated with the mainshock asperity. Normal-fault-type aftershocks were widely distributed in the overriding
plate and the outer-rise of the Pacific Plate. These aftershocks may have been due to a tensional stress change
caused by the coseismic slip. Thrust-fault-type aftershocks in the subducting Pacific Plate were also interpreted
as being due to compressional stress change as a result of the coseismic slip.
关键词：Foreshock ;aftershock ;centroid moment tensor ;focal mechanism
